expand_avrule_helper: cleanup

General clean up for expand_avrule_helper:
1. Minimize the conversions of AVRULE specification to AVTAB specification,
   they are almost the same, the one exception is AVRULE_DONTAUDIT.
2. Clean up the if/else logic, collapse with a switch.
3. Move xperms allocation and manipulation to its own helper.
4. Only write avkey for values that change.
5. Return error rather than assert on invalid specification.

Signed-off-by: William Roberts <william.c.roberts@intel.com>
1 file changed